Catalyst for reducing mercury, a mercury conversion unit, and an apparatus for measuring total mercury in combustion exhaust gas by using the same

ABSTRACT

The present invention relates to a catalyst for reducing mercury, which comprises a reagent comprising any of the sulfites of potassium, sodium, calcium and magnesium, or any of the phosphates thereof, or a combination of them, as a main reagent of a catalyst component. And the present invention relates to the catalyst for reducing mercury, wherein the catalyst component is mixed with a different salt as an agent for inhibiting crystallization of the catalyst component.

RELATED APPLICATIONS

This is a divisional application of U.S. Ser. No. 11/724,540, filed onMar. 15, 2007, which claims priority from Japanese Application No.2006-097630 filed Mar. 31, 2006.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a catalyst for reducing mercury, amercury conversion unit and an apparatus for measuring total mercury inexhaust gas by using the same, and relates in particular to a catalystfor reducing mercury, a mercury conversion unit and an apparatus formeasuring total mercury in exhaust gas by using the same, wherein totalmercury in coal combustion exhaust gas for example is measured.

2. Description of the Related Art

As an apparatus for measuring total metal mercury in combustion exhaustgas, there has been conventionally used an apparatus for measuring totalmercury for a fixed source by using a continuous measurement method or adilution measurement method of using a gold amalgamcatching/concentrating operation, stipulated under JIS K 0222. Thedilution measurement method of using gold amalgam is a method ofmeasuring metal mercury, which comprises heating a sample gas at hightemperatures to reduce a mercury compound into metal mercury, thendiluting it to catch mercury as gold amalgam, and after a predeterminedtime, re-gasifying amalgam mercury at high temperatures, and measuringmetal mercury by a ultraviolet absorption method (see for example JIS K0222-1997).

As applications are expanded in recent years, however, conventionalmethods of measuring mercury in, for example, exhaust gas fromcombustors are influenced by the presence of nitrogen oxides (NOx),sulfur dioxide (SO₂) or hydrogen chloride (HCl) in the exhaust gas, andthus it is difficult to obtain sufficiently accurate measurement values.At the request of improvement of measurement methods or of newmeasurement methods, the following various proposals are made atpresent.

Specifically, as shown in FIG. 7, there is proposed a method ofcontinuously analyzing gaseous total mercury contained in exhaust gasupon treatment of sludge and wastes, wherein a mercury-containing gas isheated (about 230° C.) if necessary and then the mercury-containing gasis treated in a gaseous form with a heated (about 200° C.) solidreduction catalyst 21 consisting of a metal (metal tin, metal zinc etc.)thereby reducing a mercury compound (mercury chloride, mercury oxideetc.) in the mercury-containing gas into metal mercury which is thenmeasured with a flameless atomic absorption spectrometer 22 (see, forexample, JP-B 1-54655).

In an apparatus 31 for analyzing mercury in a mercuricchloride-containing gas, as shown in FIGS. 8(A) and (B), a reducingagent 34 comprising a stannous chloride coating 33 formed on the surfaceof tin particle 32 is charged into a reduction reactor 35, and by areduction apparatus 36, the gas is passed through the reduction reactor35, whereby Hg²⁺ in mercuric chloride is reduced to Hg⁰ by the reducingagent 34, and the reduced Hg⁰ is analyzed by an analyzer (flamelessatomic absorption spectrometer) 37. By doing so, mercury analysis can beproperly carried out even if the concentration of mercuric chloride inthe gas is low (see, for example, JP-A 2001-33434).

However, when the measurement methods or measuring apparatuses describedabove are used to measure total mercury in coal combustion exhaust gas,accurate measurement is difficult because of poisoning of the catalystby metal oxides such as selenium oxide and arsenic oxide (both of whichare gases) coexisting in exhaust gas and the influence of coexisting gascomponents SO₂, NO₂ and water on the catalytic activity.

That is, it was found through the inventors' verification that in aprocess of reduction treatment of mercury compounds (divalent mercury),metal oxides undergo reduction reaction simultaneously with the reactionof the mercury compounds, to easily form amalgam with mercury and catchmercury, thus gradually lowering measurement values and making mercurymeasurement infeasible in some cases. Particularly, coal combustionexhaust gas contains a relatively large amount of metal oxides such asthose of lead (Pb) and selenium (Se) which easily form amalgam withmercury, and their influence cannot be negligible and their avoidance isdifficult by the conventional methods.

The dilution measurement method of using a gold amalgamcatching/concentrating operation prescribed in JIS K 0222 supra hasproblems such as significant errors in dilution, limitation to batchmeasurement, and deterioration in performance of high-temperaturereduction catalyst. This conventional method makes use of ahigh-temperature catalyst, but there is also a problem of necessity forarrangement of an acid scrubber because SO₂ is oxidized at hightemperatures to form SO₃ mist. Further, element mercury is easilyoxidized again with gas-contacting materials (for example, stainlesssteel (SUS)) used for the high-temperature catalyst, so the selection ofa material constituting the catalyst unit is necessary.

As described above, there are some demands for the apparatus formeasuring total mercury, which is directed to coal combustion exhaustgas, but the apparatus for continuously measuring mercury by anextraction sampling system other than the dilution method of using agold amalgam/concentrating operation is substantially not developedunder the present circumstances.

In the atomic absorption spectrometry, photoabsorption in theultraviolet range is utilized, and thus the interference influence ofSO₂ and NO₂ coexistent at a high concentration of several thousand ppmin coal combustion exhaust gas cannot be negligible.

To cope with such demand, the object of the invention is to provide acatalyst for reducing mercury and a mercury conversion unit, which canmaintain high reducing function even if various metal oxides andstrongly acidic corrosive gases are coexistent such as in coalcombustion exhaust gas. The present invention also provides an apparatusfor measuring total mercury comprising the catalyst for reducing mercuryand the mercury conversion unit, which is capable of continuousmeasurement with high accuracy and stability for a long time withoutundergoing the influence of coexisting components.

SUMMARY OF THE INVENTION

The present inventors made extensive study, and as a result they foundthat the above object can be achieved by a catalyst for reducingmercury, a mercury conversion unit and an apparatus for measuring totalmercury in exhaust gas comprising the same, and the present inventionwas thereby completed.

The present invention relates to a catalyst for reducing mercury, whichcomprises a reagent comprising any of the sulfites of potassium, sodium,calcium and magnesium, or any of the phosphates thereof, or acombination of them, as a main reagent of a catalyst component.

As described above, when various metal oxides or strongly acidiccorrosive gases such as in coal combustion exhaust gas are coexistent,continuous measurement with catalyst performance maintained withoutundergoing the influence of coexisting components is hardly feasiblewith the conventional apparatus for measuring total mercury. That is,the catalyst for reducing mercury is required to exhibit not only (A)selectivity reductive action for a mercury compound (bifunctional), butalso (B) unreactivity with metal oxides, particularly a property ofhardly forming amalgam with lead (Pb) and selenium (Se), and (C)corrosion resistance to strongly acidic gas. The present inventorexamined various catalysts having a function of reducing mercury withoutundergoing such chemical influence, and as a result they found that acatalyst component comprising a reagent (reagent of the presentinvention) comprising any of the sulfites of potassium, sodium, calciumand magnesium, or any of the phosphates thereof, or a combination ofthem, as a main reagent of a catalyst component is very useful forfunction of reducing mercury.

That is, (A) with respect to the selectivity, a catalyst componentcomprising the reagent of the present invention as a main reagent reactswith a mercury compound (bifunctional) thereby exerting a reducingaction selectively on the mercury compound. It was also found that thereagent of the present invention is poor in reactivity with acidicsubstances, thus eliminating the poisoning action, on the catalyst, ofacidic substances such as SO₂ and NO₂ contained in a large amount incoal combustion exhaust gas. The present invention can secure a highlyselective catalyst for reducing mercury by utilizing these findings.

Specific examples of the reductive reaction can include the reaction ofmercury chloride (HgCl₂) with K or Na sulfite (M₂SO₃) as shown below.The details will be described later.

HgCl₂+M₂SO₃→Hg+2MCl+SO₂+½O₂ (300 to 500° C.)

wherein M represents K or Na.

(B) Non-reactivity with the metal oxide is virtually not influenced inexperiments. (C) The corrosion resistance is a property inherent in thereagent of the present invention and is not problematic in experiments.In the present invention, the properties of the reagent of the presentinvention described above (A)-(C) are utilized thereby providing amercury reducing catalyst capable of maintaining high reducingperformance.

The present invention relates to the catalyst for reducing mercurydescribed above, wherein the catalyst component is mixed with adifferent salt as an agent for inhibiting crystallization of thecatalyst component.

The reagent of the present invention comprises as a main reagent awater-soluble compound such as sulfite and phosphate, and thereforewater if present can act as water of crystallization to causecrystallization of the catalyst component. When crystallization occurs,the resistance of a layer of the catalyst to gas passage is increased,and the efficiency of reduction may be reduced. Through inventor'sverification, on one hand, it was found that when the reagent consistsof one component or similar salts, crystallization of the reagent easilyoccurs, but the reagent when mixed with a different salt is hardlycrystallized. According to the present invention based on this finding,the reagent of the present invention and a basic salt that is adifferent salt serving as a crystallization inhibitor are mixed with thecatalyst component, whereby recrystallization at high temperatures isprevented and a large reaction surface area can be maintained. Thus, amercury reduction catalyst capable of maintaining high reducingperformance can be provided even for gas containing a large amount ofwater, such as coal combustion exhaust gas. As used herein, the“different salts” refer to salts different in crystalline structure. Forexample, inorganic sodium sulfite (Na₂SO₃) has a hexagonal system, andas the different salts correspond thereto, such as calcium carbonate(CaCO₃) has a trigonal or orthorhombic system, calcium sulfate (CaSO₄)has an orthorhombic system, and barium carbonate (BaCO₃) has a calcitestructure.

The present invention relates to the catalyst for reducing mercurydescribed above, wherein the catalyst component comprising the reagentas a main reagent is supported by a basic binder with an inorganicporous particle material as a carrier of the catalyst.

A major factor for determining catalyst activity includes properties ofthe reagent forming a catalyst, the surface area, etc. When exhaust gassuch as coal combustion exhaust gas is a subject of measurement, thesample often contains a large amount of dust and mist, and how theeffective surface area of the catalyst is secured is important for useof the mercury reduction catalyst for a long time. On the other hand,the reagent of the present invention is originally in a powdery form,and as a result of examination of how the reagent is converted into aneasily handled catalyst with high efficiency, a mercury reductioncatalyst securing the surface area of the catalyst, preventing abrasionof the catalyst and maintaining high reducing function for a long timecould be provided by supporting the reagent by a basic binder with aninorganic porous particle material as a carrier of the catalyst.

The present invention relates to the catalyst for reducing mercurydescribed above, wherein refractory and/or activated alumina is used asthe inorganic porous particle material, and liquid glass and/or lithiumsilicate is used as the basic binder.

By the verification of the present invention described above, it wasfound that supporting the catalyst by the basic binder with an inorganicporous particle as a carrier of the catalyst is effective for securingthe surface area of the catalyst, preventing abrasion of the catalystand maintaining high reducing function of a long time. As a result offurther examination of the inorganic porous particle material and thebasic binder, it was found that the catalyst, wherein refractory and/oractivated alumina is used as the inorganic porous particle material andliquid glass and/or lithium silicate is used as the basic binder, isformed into particles or in a honeycomb form, whereby the surface areaof the catalyst can be secured while abrasion of the catalyst isprevented, and its reducing function can be maintained for a long time.

The present invention relates to a mercury conversion unit comprisingthe above catalyst for reducing mercury charged into a predeterminedcontainer consisting of an inorganic material such as glass, quartz orceramics or an oxidized metal such as oxidized stainless steel ortitanium as a gas-contacting material wherein the operative temperatureof the catalyst for reducing mercury is 300 to 500° C.

Generally, the reductive reaction is rendered more reactive as thetemperature is increased, and the catalytic action is influencedsignificantly by the temperature, and thus a predetermined temperatureis preferably maintained. As a result of verification, it was found thatan operative temperature of 300° C. or more is preferable for securingand maintaining a predetermined efficiency of reduction with the reagentof the present invention as a catalyst for reducing mercury. Under acondition of further higher temperatures, on the other hand, there arisesecondary problems such as formation of SO₃, so it was found that theupper limit of operative temperature is preferably about 500° C. It wasfound that as the gas-contacting material charged with the catalyst,materials such as SUS allow reduced mercury to be oxidized again, whileinorganic materials such as glass, quartz and ceramics or metals such asoxidized SUS, titanium (Ti) etc. are preferable. According to thepresent invention, a mercury conversion unit wherein the operativetemperature of the catalyst for reducing mercury is maintained in apredetermined range and the gas-contacting material is selected can beprovided and applied to an apparatus for measuring total mercury inexhaust gas, which is capable of continuous measurement with highaccuracy and stability for a long time without undergoing the influenceof coexisting components.

The present invention relates to an apparatus for measuring totalmercury using the above catalyst for reducing mercury or the abovemercury conversion unit, which has the catalyst for reducing mercury orthe mercury conversion unit in a part of a sample collection flow path,and after the treatment, a sample is introduced into an ultravioletabsorption analyzer.

As described above, the catalyst for reducing mercury or the mercuryconversion unit according to the present invention has very excellentfunctions such as maintenance of high reducing function even in thecoexistence of various metal oxides and strongly acidic corrosive gases.Accordingly, a very excellent apparatus for measuring total mercury canbe constituted by applying such functions to the apparatus for measuringtotal mercury in exhaust gas, wherein the catalyst for reducing mercuryor the mercury conversion unit is arranged in a part of a samplecollection flow path, and a mercury compound in a sample is reduced andconverted into element mercury which is then measured by an ultravioletabsorption analyzer. Particularly by using an ultraviolet absorptionanalyzer having selectivity for element mercury, an apparatus formeasuring total mercury in exhaust gas, which is capable of continuousmeasurement with high accuracy and stability for a long time withoutbeing influenced by coexisting components, can be provided.

The present invention relates to the apparatus for measuring totalmercury in exhaust gas, wherein a mist capturing agent or a counteragentis used in a pre-stage for the catalyst for reducing mercury or themercury conversion unit in the sample collection flow path.

As described above, the catalyst for reducing mercury or the mercuryconversion unit according to the present invention is used by limitingthe operative temperature (500° C. or less) such that the reducingfunction is maintained for a long time. However, when a sample itselfcontains a substance such as SO₃ mist or oil mist causing corrosion ofthe sample collection flow path or poisoning the catalyst, limitation ofthe operative temperature cannot be said to be satisfactory. In suchcase, the present invention makes use of a mist capturing agent or acounteragent in a pre-stage for the catalyst for reducing mercury or themercury conversion unit in the sample collection flow path, whereby thefunctions of the catalyst for reducing mercury or the mercury conversionunit can be maintained for a long time to enable continuous measurementwith high accuracy and stability for a long time.

According to the present invention, there can be provided a catalyst forreducing mercury and a mercury conversion unit capable of maintaininghigh reducing function even in the coexistence of various metal oxidesand strongly acidic corrosive gases such as coal combustion exhaust gas,which has conventionally been difficult as described above. There canalso be provided an apparatus for measuring total mercury in exhaustgas, which is capable of continuous measurement with high accuracy andstability for a long time without being influenced by coexistingcomponents.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an illustration showing temperature characteristics forefficiency of reduction with the reagent of the present invention;

FIG. 2 is an illustration showing the influence of coexistent cesiumoxide (SeO₂) on the efficiency of reduction of Hg;

FIG. 3 is an illustration schematically showing the constitution of amercury conversion unit;

FIG. 4 is an illustration schematically showing a procedure ofgranulating a mercury reduction catalyst using sulfite;

FIG. 5 is an illustration showing a constitution of an apparatus formeasuring total mercury in exhaust gas;

FIG. 6 is an illustration showing another constitution of an apparatusfor measuring total mercury in exhaust gas;

FIG. 7 is an illustration showing a constitution of a conventionalmethod of continuously analyzing gaseous total mercury in exhaust gas;and

FIG. 8 is another illustration showing a constitution of a conventionalmethod of continuously analyzing gaseous total mercury in exhaust gas.

In the drawings, 1 is a mercury conversion unit; 2, a container; 3, acatalyst for reducing mercury; 4, a heating means; 10, an ultravioletabsorption analyzer; 11, a purification apparatus; and 18, a scrubberunit.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, embodiments of the present invention are described in moredetail by reference to the Drawings.

<Fundamental Constitution of the Mercury Reduction Catalyst According tothe Present Invention>

The fundamental constitution of the mercury reduction catalyst accordingto the present invention is that the reagent of the present invention(corresponding to “a reagent comprising any of the sulfites ofpotassium, sodium, calcium and magnesium, or any of the phosphatesthereof, or a combination of them”) is contained as a main reagent ofthe catalyst component. That is, the reagent of the present inventionexerts reducing action selectively on a mercury compound (bifunctional)to covert it into element mercury, by utilizing the uniquecharacteristics: (A) selectivity of reducing catalyst action for amercury compound (bifunctional) and (B) unreactivity with coexistingacidic substances. The reagent of the present invention is poor in (C)reactivity with acidic substance such as SO₂ and NO₂, to eliminate thepoisoning action thereof on the catalyst (poisoning eliminatingfunction). The catalyst for reducing mercury according to the presentinvention refers broadly to a mercury reduction catalyst comprising thereagent of the invention as a main reagent to which a substancefunctioning as a co-catalyst and an agent for inhibiting crystallizationof the catalyst component are added to improve characteristics of thecatalyst for reducing mercury.

Hereinafter, what is verified for (A) selectivity of reducing catalysisaction, (B) unreactivity with metal oxides and (C) poisoning eliminationfunction, which are required of the catalyst for reducing mercury, isdescribed on the basis of the functions of the reagent of the presentinvention.

(A) Reducing Catalyst Function of the Reagent of the Present Invention

For the reducing catalyst function of the reagent of the presentinvention, the selectivity of reduction action for a mercury compound(bifunctional) is required. Now, the reduction of HgCl₂ as a majorcomponent in coal combustion exhaust gas by the catalyst action isdescribed.

(A-1) Principle of Operation of Reductive Catalyst Reaction

It is considered that in a solid-gas reaction system, reductive reactionof mercury proceeds to such a state that a more sable compound isformed. That is, in the reaction of HgCl₂, except for high-temperatureconditions of 600° C. or more, the degree of reactivity of the reagentof the present invention with the mercury compound (bifunctional)involves in the reactivity of the cation constituting the reagent of thepresent invention with chlorine (Cl) constituting HgCl₂ and thereactivity of the anion constituting the reagent of the presentinvention with Hg. Accordingly, the progress of the reaction can bejudged by comparing the estimated formation system with the energy offormation of the substance to be formed (ΔH, kJ/mol) and by analyzingthe degree of the energy of formation of the acting substance.

(A-2) Type of the Reagent of the Present Invention

As a result of verification based on the analysis described above, itwas found that the reagent of the present invention serving as a mainreagent of the catalyst component is preferably a heat-resistantreagent, and the cation is preferably a salt of an alkali metal such aspotassium (K) or sodium (Na) or an alkaline earth metal such as calcium(Ca) or magnesium (Mg). It was found that the anion is particularlypreferably sulfites or phosphates. Specific reagents composed of suchcombinations are shown in Table 1.

TABLE 1 Reagent Sulfites Potassium Sulfite (K₂SO₃) Sodium Sulfite(Na₂SO₃) Calcium Sulfite (CaSO₃) Magnesium Sulfite (MgSO₃) PhosphatesPotassium Phosphate (K₃PO₄) Sodium Phosphate (Na₃PO₄) Calcium Phosphate(Ca₃(PO₄)₂) Magnesium Phosphate (Mg₃(PO₄)₂)

(A-3) Evaluation Test

With respect to the salts mentioned as the reagent of the presentinvention, the analysis in (A-1) was validated in a verification test togive the following new findings:

(a) Test Method

(a-1) At a temperature of 300 to 500° C., a standard solution ofmercuric chloride (HgCl₂) is gasified and passed through the catalystcolumn to determine the recovery of Hg.(a-2) In the reagent of the present invention, special-grade reagentswere used, and for the reagent of the present invention in a powderyform, refractory porous particles were used after dilution to about 30%(wt/wt). Na₂SO₃ was used by adhering it to the porous particlesubstance.(a-3) Since SO₂ is contained at high concentration in exhaust gas, thereactivity of the reagent of the present invention with SO₂ or thestability of the reaction product can influence the reactivity of thereagent of the present invention with HgCl₂, thereby influencing theefficiency of reduction with the reagent of the present invention. Thiswas verified by the method (a-1) at a reaction temperature of 400° C. inthe coexistence of SO₂.

(b) Test Results

(b-1) With Respect to Sulfites

Effective efficiency of reduction could be achieved by various sulfitessuch as alkali metal salts Na₂SO₃ and K₂SO₃ and alkaline earth metalsalts CaSO₃ and MgSO₃. Specifically, 95% or more efficiency of reductionwas confirmed by every sulfite at 400° C. at SV=1000 hr⁻¹. The sameresults could also be obtained by using a mixture thereof.

(b-2) With Respect to Phosphates

As effective compounds other than the sulfites, phosphates Na₃PO₄ andK₃PO₄ and Ca₃(PO₄)₂ and Mg₃(PO₄)₂ were revealed to exhibit 95% or moreefficiency of reduction of HgCl₂ in the same temperature range.

(b-3) With Respect to Other Salts Stable at High Temperatures

It was found that salts stable at high temperatures such as sodiumsulfate (Na₂SO₄), NaCl, KCl, calcium chloride (CaCl₂), barium chloride(BaCl₂) do not react at all.

(b-4) With Respect to Reductive Reaction in the Coexistence of SO₂

It was found that in the coexistence of SO₂ at a reaction temperature of400° C., Na₂SO₃, K₂SO₃, CaSO₃ and MgSO₃ contribute to the reductivereaction of HgCl₂.

(A-4) Mechanism of Reductive Reaction

(a) On the basis of the findings described above, the mechanism of thereductive reaction is organized below. The salts were classified into 2groups: (a-1,2) alkali meal (M: for example, Na, K) sulfite (M₂SO₃) andphosphate (M₃PO₄) and (a-3,4) alkaline earth metal (M′: for example, Ca,Mg) sulfite (M′SO₃) and phosphate (M′₃(PO₄)₂). The reaction temperaturewas 400° C.

HgCl₂+M₂SO₃→Hg+2MCl+SO₂+½O₂(300 to 500° C.)  (a-1) Reaction formula:

wherein M represents K or Na. For example, more stable potassiumchloride (KCl) is formed by reaction with potassium sulfite (K₂SO₃), andsodium chloride (NaCl) is formed by reaction with sodium sulfite(Na₂SO₃).

3HgCl₂+2M₃PO₄→3Hg+6MCl+P₂O₅+3/2O₂(300 to 500° C.)  (a-2) Reactionformula

wherein M represents K or Na. For example, more stable potassiumchloride (KCl) is formed by reaction with potassium phosphate (K₃PO₄),and sodium chloride (NaCl) is formed by reaction with sodium phosphate(Na₃PO₄)

HgCl₂+M′SO₃→Hg+2M′Cl₂+SO₂+½O₂(300 to 500° C.)  (a-3) Reaction formula

wherein M′ represents Ca or Mg. For example, more stable calciumchloride (CaCl₂) is formed by reaction with calcium sulfite (CaSO₃), andmagnesium chloride (MgCl₂) is formed by reaction with magnesium sulfite(MgSO₃).

3HgCl₂+M′₃(PO₄)₂→3Hg+3M′Cl₂+P₂O₅+3/2O₂(300 to 500° C.)  (a-4) Reactionformula

wherein M′ represents Ca or Mg. For example, more stable calciumchloride (CaCl₂) is formed by reaction with calcium phosphate(Ca₃(PO₄)₂), and magnesium chloride (MgCl₂) is formed by reaction withmagnesium phosphate (Mg₃(PO₄)₂).

(b) Acidic substances such as SO₂, NO₂, chlorine compound etc. arecontained in a large mount in coal combustion exhaust gas, so when thereduction catalyst has a reduction action on SO₂, NO₂ etc., the functionof reduction of Hg²⁺ is substantially caused. In the catalyst such asinorganic carbonate, the reaction in which the basal part constitutingthe carbonate is replaced by a halide or nitrate proceeds gradually, andin use for a long time, the poisoning action of acidic substances on thecatalyst is made significant and the reductive reaction characteristicsare deteriorated.

(A-5) With Respect to an Indicator of the Reductive Reactivity of Salts

In view of the verification results described above, a specificindicator for judging whether salts have reductive reactivity or not caninclude ionic dissociation constant (pKa) (in an aqueous solution).Specifically, from the relationship with the acid dissociation constantof a typical compound shown in Table 2, the reductive reactivity ofsalts can be estimated to be high when pKa≦5, as shown in the thickframe of Table 2. For example, this corresponds to pK value of 2.15 ofphosphate against that of 1.91 of sulfite, and it is also proven thatfrom the results wherein Na₃PO₄ or K₃PO₄ attained 95% or more efficiencyof reduction, as described above.

TABLE 2 Acid Dissociation Constant (pKa) Element as a Main H₂CO₃ 6.35Reagent HClO 7.53 HCN 9.22 H₂CrO₄ 1.50 HF 3.17 H₂MoO₄ 3.62 H₃PO₄ 2.15H₂SO₃ 1.91 H₂SeO₃ 2.75 H₂SiO₂ 9.93

(A-6) Catalyst Temperature and Reduction Efficiency

As described above, an Hg—Cl bond should be cleaved in order that thereagent of the present invention functions as a mercury reductioncatalyst. That is, supply of dissociation energy for cleaving this bond,particularly energy not lower than the binding energy, are regarded asnecessary. Theoretically, Hg²⁺ is said to be thermally decomposed at600° C. or more, thereby partially forming element mercury. The reactionis said to proceed stoichiometrically, but experimentally, the reactionrate is low, and for using the reagent in a mercury conversion unit byheat decomposition for pretreatment in an apparatus for measuring totalmercury, use under a high-temperature condition at 850 to 900° C. isnecessary. Accordingly, the efficiency of reduction of HgCl₂ wasmonitored with temperature as an indicator when the reagent of thepresent invention functions as the mercury reduction catalyst. As shownin FIG. 1, the result of verification using the reagents of theinvention K₂SO₃ and Na₂SO₃ in the vicinity of SV=1000 hr⁻¹ revealed 80%or more efficiency at 300° C. or more. With respect to CaSO₃, MgSO₃,Na₃PO₄ and K₃PO₄ or Ca₃(PO₄)₂ and Mg₃(PO₄)₂, the same results wereconfirmed at SV=1000 hr⁻¹. Under high-temperature conditions for morethan 500° C., however, there arise secondary problems such asdeterioration of sulfites, and thus the upper temperature in actualoperation is preferably about 500° C. for maintaining the stability ofsulfites. That is, it was found that a temperature of about 300 to 500°C. is preferable for a means of reductive reaction of a mercury compoundsuch as HgCl₂ in the mercury conversion unit using the reagent of thepresent invention. The operative temperature of the catalyst is in themiddle temperature range (300 to 500° C.) in which Hg²⁺ can be selectedreduced without reducing selenium oxide (SeO₂) and simultaneously thelife of the catalyst can be significantly prolonged without undergoingthe poisoning action of SO₂ gas etc.

(B) Unreactivity with Metal Oxides

Whereas reduction of a mercury compound into element mercury isessential for the apparatus for measuring total mercury in exhaust gas,as described above, metal oxides such as those of Pb and Se arecontained in a relatively large amount in exhaust gas such as coalcombustion exhaust gas, and thus in the process for reduction treatmentof a mercury compound, reductive reaction with metal oxidessimultaneously occurs to produce amalgam with mercury, thereby catchingmercury to make accurate mercury measurement infeasible. That is, themercury reduction catalyst is required to have unreactivity with suchmetal oxides, particularly a property of hardly forming amalgam with Pb,Se etc. The mercury reduction catalyst of the invention comprising thereagent of the invention as a main reagent could be proven to have veryexcellent unreactivity with metal oxides, as compared with that of theconventional catalyst. Particularly problematic SeO₂ was examined in aHg reduction efficiency test in the coexistence of HgCl₂ and SeO₂.

(B-1) Test Method

The difference in the efficiency of reduction of HgCl₂ between the statewhere SeO₂ is absent and the state where SeO₂ is coexistent wasverified. The catalyst temperature was 400° C., and the reagent of theinvention, K₂SO₃, was used as the catalyst for reducing mercury. Astandard gas previously examined for the concentration of HgCl₂generated therefrom was prepared and introduced into the catalyst toconfirm the efficiency of reduction of Hg. Then, predetermined amountsof HgCl₂ and SeO₂ were mixed to prepare a sample gas examined for theconcentration of each gas generated therefrom (50 μg/m³ HgCl₂ as astandard with a varying concentration of SeO₂), and the sample gas wasintroduced into the catalyst in the coexistence of the two, and theinfluence of SeO₂ concentration on the efficiency of reduction of Hg wastested.

(B-2) Test Results

The influence of SeO₂ concentration on the efficiency of reduction of Hgis also shown in FIG. 2. As compared with the efficiency of reduction ofHgCl₂ without SeO₂, the efficiency in the coexistence of SeO₂ was notobserved to be significantly lowered. It was found that even in the SeO₂coexistent state, the function of reduction of HgCl₂ was not influenced.

(C) Poisoning Elimination Function of the Catalyst

Strongly acidic gases such as SO₂ contained in coal combustion exhaustgas can give a long-term poisoning action on the catalyst activity. Onthe other hand, the present inventors had found that “salts stable athigh temperatures” such as phosphates and sulfites or basic substancesforming sulfites can suppress such poisoning for a long time. That is,in the present invention, the reagent of the invention itself has suchpoisoning elimination function by which the high reducing function ofthe catalyst for reducing mercury can be maintained. Furthermore, thereagent of the invention is a chemically and physically stable compoundeven at 300 to 500° C. and is thus suitable as a catalyst for reducingmercury.

From the verification data shown above, it was found that a compoundwhose pKa is significantly deviated from 2, in the range of 300 to 500°C., easily causes the neutralization with strongly acidic gases such asSO₂ or is easily decomposed. That is, salts with pKa≦5 or pKa in thevicinity of 2 or less are suitable as catalyst substances effectingreductive reaction of HgCl₂ without undergoing the influence of stronglyacidic gases contained in coal combustion exhaust gas, and arespecifically substances represented by salts such as sulfites andphosphates.

Poisoning inhibitors against strongly acidic gases such as SO₂ arepreferably those comprising phosphates or sulfites as a main reagentmixed with one or more basic substances forming sulfites. By using thereagent mixed with such two or more salts stable at high temperatures,the poisoning inhibiting effect on strongly acidic gases such as NO₂ orCl₂ other than SO₂ coexistent in exhaust gas can be expected.

<Countermeasures for Suppressing Catalyst Crystallization>

The reagent of the present invention is based on a water-solublecompound such as sulfite and phosphate, and thus the presence of waterin a sample can serve as water of crystallization to causecrystallization or recrystallization of the catalyst component. Whencrystallization occurs, the resistance of the catalyst layer to gaspassage is increased, and the efficiency of reduction may be reduced.Against this, the present inventors found that the crystallization orrecrystallization of the reagent comprising one component or similarsalts easily occur, but hardly occurs where different salts are mixed.That is, as an agent inhibiting crystallization of the reagent of theinvention, a basic salt that is a different kind of salt from thereagent of the present invention are mixed with the catalyst component,whereby recrystallization at high temperatures is prevented, a largereaction surface area can be maintained, and when a different kind ofsalt stable at high temperatures is partially mixed, recrystallizationhardly occurs, and the effective area of the sulfite catalyst can bemaintained for a long time. As the different kind of salt, basic saltssuch as Na salt and Ca salt are effective against K.

<Mercury Conversion Unit>

The mercury reduction catalyst 3 prepared by the treatment describedabove is charged into a container 2 for using a mercury conversion unit1 illustrated in FIG. 3, and is arranged in, for example, a sampletreatment flow path of an apparatus for measuring total mercury inexhaust gas described later. The container 2 is composed of a strong,corrosion- and heat-resistant material. As the gas-contacting material,a material such as SUS causes reduced mercury to be easily oxidized, andthus inorganic materials such as glass, quartz and ceramics arepreferable, and the metal is preferably oxidized SUS or Ti. When thereis a large amount of dust etc. in exhaust gas, a dust-removing filter(not shown) can be arranged. The mercury conversion unit 1 is providedwith a heating means 4 for adjusting the mercury reduction catalyst 3 tothe optimum temperature to secure desired efficiency of reduction ofmercury. When the mercury conversion unit is used in the apparatus formeasuring total mercury in coal combustion exhaust gas, the reagent ofthe present invention is charged as the mercury reduction catalyst 3,and the operative temperature is maintained at 300 to 500° C.

Since the reagent of the present invention is originally powder, thereagent is compacted in the form of particles or granules. That is, thereagent is supported by a basic binder with an inorganic porous particlesubstance as a carrier for the catalyst and compacted in the form ofparticles or granules, thereby securing the surface area of thecatalyst, preventing abrasion of the catalyst and maintaining highreducing function for a long time. In the following section, the methodfor this is described in detail.

<Method of Forming Granules or Particles of the Catalyst>

The reagent of the present invention is in a powdery form and isinherently easily crystallized by retaining crystal water as describedabove, so a method of impregnating a carrier with the reagent dissolvedin an aqueous solution is not suitable in the conventional method offorming granules or particles. That is, in the present invention,refractory and/or activated alumina is used as the inorganic porousparticle material, and a basic binder such as liquid glass and lithiumsilicate is used as the binder, whereby catalyst particles or granuleseasily handled as compact can be prepared while the surface activity ofthe reagent of the present invention is maintained.

(A) Method of Granulation of the Catalyst Substance

10 to 30% by weight of the reagent of the invention that is any ofsulfites or a mixture thereof is impregnated into or adhered to aninorganic porous substance as a carrier.

(A-1) Selection of Inorganic Porous Substance

Such as Pamister (trade name: Ohe Chemicals inc.), activated alumina,activated carbon, and molecular sieve can be used as the carrier, andactivated carbon and molecular sieve have catalyst performancethemselves. These materials are selected on the basis of thecharacteristics of the reagent of the present invention and the binder.In the case of sulfite, for example, Pamister is preferable from theviewpoint of a larger amount of its adhering powder and higherretention.

(A-2) Method of Adhering to the Inorganic Porous Substance

Because any of the reagents of the present invention is water-soluble,water glass (chemical formula: Na₂O.nSiO₂.xH₂O), lithium silicate(chemical formula: Li₂O.nSiO₂.xH₂O) etc. are used as basic binders forpreventing air oxidation by dissolution. As lithium silicate, there aretypes 35, 45 and 75 (manufactured by Nissan Chemical Industries, Ltd.).

(B) Operation of Granulation

For example, the operation, wherein K₂SO₃ as the reagent of theinvention, lithium silicate as the binder, and Pamister as the carrierare used in granulation, is illustrated in FIG. 4(A), and the state ofthe particles is illustrated in FIG. 4(B). In this case, K₂SO₃3c ispreferably adhered in a dry state, because when it is air-oxidized in astate dissolved in water, it becomes a sulfate and loses mercuryreduction performance. Specific procedures for granulation are asfollows:

(1) Pamister 3 a is prepared. Pamister heat-treated at 600 to 800° C.for 6 hours or more in pretreatment is used.(2) Pamister 3 a is impregnated with lithium silicate. As shown in FIG.4(B), a thin layer 3 b of lithium silicate is formed on the surface ofPamister 3 a.(3) K₂SO₃3c is added to, and mixed with, Pamister 3 a impregnated withlithium silicate. As shown in FIG. 4(B), K₂SO₃3c is adhered almostuniformly to the surface of the thin layer 3 b of lithium silicate.(4) Immediately after mixing, the product is placed in a thermostaticchamber and vacuum-dried at ordinary temperature for about 1 hour.Thereafter, it is vacuum-dried at 50° C.(5) The product is dried in a vacuum dryer at 150° C. for 12 hours. Asshown in FIG. 4(B), particles having K₂SO₃3c formed thereon arecompleted.

<Constitution of the Apparatus for Measuring Total Mercury in ExhaustGas>

FIG. 5 illustrates one constitution of the apparatus for measuring totalmercury in exhaust gas by using the above catalyst for reducing mercuryor the above mercury conversion unit 1. This constitution is suitablefor the case where the subject of measurement is total mercury(Hg²⁺+Hg⁰) as a plurality of components containing the same element,such as divalent mercury (Hg²⁺) and element mercury (Hg⁰) which can beconverted into each other. That is, Hg²⁺ in the sample gas is firstconverted into Hg⁰ as the object of measurement of the total amount ofHg⁰, by the catalyst for reducing mercury or the mercury conversion unit1, and by comparison with a gas obtained by selectively eliminating Hg⁰,the influence of other existing components and the background can beeliminated. Hereinafter, the present invention is described by referenceto the apparatus for measuring total mercury wherein the ultravioletabsorption analyzer 10 is used as a measurement means.

This constitution directed to Hg²⁺ and Hg⁰ as the object of measurementincludes:

(1) as a sample treatment means, a mercury convention unit 1 forconverting Hg²⁺ selectively into Hg⁰ and a purification apparatus 11 forselectively removing Hg⁰ setting in one flow path c branched from asample gas flow path a,(2) as a calibration means, a means of feeding Hg²⁺-free and Hg⁰-freezero gas and a means of feeding Hg⁰ at predetermined concentration; and(3) as a measurement means, an ultraviolet absorption analyzer 10 fordetecting the concentration of Hg⁰ selectively,

wherein in an arithmetic processing means (not shown), the detectionfunction of Hg⁰, calibration function and the treatment function of asample treatment means are checked and treatment based on each functionis carried out.

A sample is suctioned and collected through a sample inlet(corresponding to a sample collection means) 13 by a suction pump 14arranged downstream of an ultraviolet absorption analyzer 10. Thecollected sample is cleaned with a dust filter 15 and then passesthrough valve V1 and then through a mercury conversion unit 1 arrangedin flow path a and is divided into halves, while in (flow path c), Hg⁰in a sample is removed by the purification apparatus 11, and the sampleis passed through a flow path resistance 16 a and introduced into theultraviolet absorption analyzer 10, whereas in (flow path b), the sampleis not treated and passed through the flow path resistances 16 b, 16 aand introduced into the ultraviolet absorption analyzer 10. The flowpath in which the absorption pump 14 was arranged is provided inparallel with a pressure regulating apparatus 14 a, thereby forming astate which can be always suctioned from the upstream side, wherebyloading on the suction pump 14 can be reduced and the suction pressurecan be stably regulated. As the gas contacting material, metals such asTi and oxidized SUS in addition to inexpensive glass, quartz andceramics can be used.

Usually, at the time of usual measurement, flow paths b and c areswitched periodically, and from a difference between the two, Hg²⁺ isdetected by an ultraviolet absorption analyzer 10. The switching betweenthe two flow paths is carried out by an electromagnetic valve Vfarranged upstream of the ultraviolet absorption analyzer 10. At the timeof zero calibration, zero gas is introduced via a calibration gas inlet17 a, passes through a flow path d and is introduced into theultraviolet absorption analyzer 10. At the time of span calibration,span gas at a predetermined concentration generated from a generator 12b into which zero gas was introduced through calibration gas inlet 12 ais introduced via flow path d into the ultraviolet absorption analyzer10. Switching of valve Vf is carried out usually in a cycle of about 0.5to 30 seconds. Measurement, calibration and checking are describeddetail below.

The ultraviolet absorption analyzer 10 (not shown) forms an opticalsystem consisting of an ultraviolet light source part, a sample cellpart, an ultraviolet detector and an optical filter, wherein theconcentration of Hg⁰ in the sample introduced into the sample cell partcan be measured by detecting the amount of light (adsorbed by Hg⁰ in thesample) in the ultraviolet region.

A purification apparatus 11 uses, for example, an adsorbent such asactivated carbon, by which Hg⁰ in the sample can be selectively adsorbedand removed. In addition, for example, Pt-silica- or Pd-alumina-basedcatalyst or a catalyst such as V₂O₅ is used to oxidize Hg⁰ in a sampleinto Hg²⁺ which cannot be detected with the ultraviolet absorptionanalyzer 10, whereby Hg⁰ can be selectively removed. When an oxidizationcatalyst is used as the purification apparatus 11, the operativetemperature can be the same middle-temperature range (for example 300 to400° C.) as in the mercury conversion unit 1, and the two can beaccommodated in the same unit to integrate of the temperature regulationmechanism and to downsize the apparatus.

Hg gas at predetermined concentration for calibration or verificationcannot be prepared as high-pressure gas, and a generator 7 should beused. For example, zero gas is passed through a surface layer of Hg keptat predetermined temperature, or Hg permeating a permeation tube dippedin an Hg liquid bath is mixed with zero gas, whereby Hg gas atpredetermined concentration can be obtained. By diluting it with zerogas, Hg gas at low concentration can be obtained.

FIG. 6 illustrates another constitution of the apparatus for measuringtotal mercury in exhaust gas, which uses the catalyst for reducingmercury or the mercury conversion unit 1 according to the presentinvention. The apparatus for measuring total mercury in exhaust gascomprises a scrubber unit 17 charged with a mist capturing agent or acounteragent arranged before a stage of the mercury conversion unit 1 ina sample collection path flow.

When substances such as SO₃ or oil mist causing corrosion of the samplecollection flow path are contained in a large amount in a sample, suchsubstances are removed with a scrubber unit 17, whereby the mercuryreduction function of the mercury conversion unit 1 is maintained for along time and is capable of continuous measurement with high accuracyand stability for a long time.

As the mist capturing agent, porous silica/alumina adsorbent is usedwhereby SO₃ mist, oil mist etc. in exhaust gas can be removed. Further,phosphoric acid has a function of improving a mist-capturing ability,and phosphoric acid is preferably adhered to the mist capturing agent.As a counteragent for strongly acidic gas, a basic substance forming asulfite functioning as the reduction catalyst can be used to remove HClor Cl₂ in exhaust gas. The concentration (content) of the counteragentcan be increased to measure high-temperature exhaust gas containing alarge amount of SO₃ mist. In this manner, one essential feature of thepresent invention lies in an excellent property of preventing poisoningaction in addition to the catalyst function, which can be applied to theapparatus for measuring total mercury in exhaust gas, to bring about anexcellent technical effect.

Measurement of total metal mercury in coal combustion exhaust gas, whichhas been difficult in the prior art, is made feasible highly accuratelyand highly sensitively with the apparatus for measuring total mercury inexhaust gas. In place of conventional batch measurement, completelycontinuous measurement can be realized. Particularly, the operativetemperature of the catalyst for reducing mercury can be in themiddle-temperature range (300 to 500° C.), whereby the reductivereaction of metal oxides in exhaust gas does not occur, and amalgam isnot generated, and thus continuous measurement of mercury in exhaust gasis not hindered. As compared with the dilution method of using goldamalgam catching/concentrating operation that is the conventionalstandard measurement method, a diluting air source or a constant flowapparatus is not necessary, and the sampling system is simple, and themaintenance is easy.

In the foregoing, the present invention has been described mainly byreference to application to a catalyst for reducing mercury in coalcombustion exhaust gas, a mercury conversion unit, and an apparatus formeasuring total mercury, but can also be applied to samples similar incomposition such as in process gas etc. or for study of variousprocesses. The present invention is particularly useful when a sample inwhich SO₂ and metal oxides are coexistent is measured.

What is claimed is:
 1. A method of reducing mercury in an exhaust gaswith a catalyst comprising the steps of: providing a reagent comprisingany of the sulfites of potassium, sodium, calcium and magnesium, or anyof the phosphates thereof, or a combination of them, as a main reagentof a catalyst component; heating the catalyst to a temperature of 300°C. to 500° C.; applying a gas sample containing a mercury compound tothe catalyst wherein the catalyst reduces Hg²⁺ into Hg⁰ with atemperature of 300° C. to 500° C.
 2. The method of reducing mercury inan exhaust gas with a catalyst according to claim 1, wherein thecatalyst component is mixed with a different salt as an agent forinhibiting crystallization of the catalyst component.
 3. The method ofreducing of mercury in an exhaust gas with a catalyst according to claim1, wherein the catalyst component comprising the reagent as a mainreagent is supported by a basic binder with an inorganic porous particlematerial as a carrier of the catalyst.
 4. The method of reducing mercuryin an exhaust gas with a catalyst according to claim 2, wherein thecatalyst component comprising the reagent as a main reagent is supportedby a basic binder with an inorganic porous particle material as acarrier of the catalyst.
 5. The method of reducing mercury in an exhaustgas with a catalyst according to claim 3, wherein refractory and/oractivated alumina is used as the inorganic porous particle material, andliquid glass and/or lithium silicate is used as the basic binder.
 6. Themethod of reducing mercury in an exhaust gas with a catalyst accordingto claim 4, wherein refractory and/or activated alumina is used as theinorganic porous particle material, and liquid glass and/or lithiumsilicate is used as the basic binder.
 7. An apparatus for measuringtotal mercury using the method of reducing mercury in an exhaust gaswith a catalyst according to claim 1, which has the catalyst forreducing mercury in a part of a sample collection flow path, and aftertreatment, a sample is introduce into an ultraviolet absorptionanalyzer.